2000 Mercruiser 7.4 mpi limited to 3900 rpm

[B.matt]

Have had this problems for a couple of years. Finally broke down and had a mercruiser mechanic look at it, he couldn't find anything wrong either. checked compression, ignition fuel press, vent valve, intake screen, fuel filter, cleaned injectors, vacuum checked. Anyone have any experience with this. The last 1/4 of throttle does nothing.

 

[Bt Doctur]

firing order wrong, 5 and 7 reversed

 

[alldodge]

Does the beeper work, as in hear 2 beeps when the key is turned to ON?

Need a scanner hooked up and get some readings if the MAP, TPS and fuel pressures stays up.
What do the plugs look like after a run? Need to run for a bit then stop quick (don't idle much) then see what they look like

 

[B.matt]

I had a mercruiser mechanic take a look at it with a scanner. He is stumped also, everything checks good. He double checked compression,fuel pressure, firing order to no avail. No fault codes stored or while running.

 

[alldodge]

Does the beeper (alarm) work?

 

[B.matt]

Yes the beeper work. No alarms when running.

 

[alldodge]

Thanks,
The scanner showed that the TPS changes as throttle opens and continues to change to 100% for the last 1/4 throttle when nothing changes?

Same for the MAP

Guess in short, was the scanner on the boat when these things were happening?

If it was, then the motor is loosing power like from a spark plug

 

[B.matt]

I was not with them when they worked on it. I will ask about the tps readings. They went back to check timing advance and I haven’t heard back yet.

 

[alldodge]

It would need to be tested on the water. Doing it on the hard doesn't provide the info needed

 

[B.matt]

Thy did take it out and ran it with the scanner

 

[B.matt]

I should add, the engine runs well just doesn’t go past 3800 rpm.

 

[alldodge]

If your motor was newer I would suspect in gear switch but you shouldn't have one